The present invention relates to an ion source equipped with an evaporation furnace for vaporizing or evaporating a solid or liquid specimen (raw material) for ionization. In particular, the invention concerns an ion source incorporating plurality of vaporizing furnaces and suited for use in an ion implantation apparatus employed in a semiconductor device manufacturing process.
When a filament is made use of as the ion source in the ion implantation apparatus of a large current capacity employed for bombarding a semiconductor substrate with ions of an impurity material by using an ion beam of a large current on the order of 10 mA, there arises a problem that the filament undergoes remarkable and rapid consumption. As an attempt to overcome this problem, ionization by thermal electrons emitted from a filament tends to be replaced by an ion source in which a plasma discharge produced under an electric field of a microwave frequency is utilized. This type ion source is referred to as the microwave discharge type ion source.
FIG. 1 of the accompanying drawings is a schematic sectional view showing a hitherto known microwave discharge type ion source. Referring to the figure, microwave 13 produced by a magnetron 8 is introduced to an accelerating electrode 10 by way of a choke flange 14 to reach an ionization chamber 2.
A magnetic field is applied to the ionization box or chamber 2 by means of an excitation coil 12, while gas of a specimen or raw material is supplied to the ionization chamber 2 through a gas supply pipe 9. As the result, a plasma is ignited within the ionization chamber 2.
An ion beam 7 is let out under the action of an extracting electrode 15 applied with an extracting voltage approximating to the ground potential. The ion beam thus extracted is used for ion implantation.
In this connection, it is known that materials or specimens which are solid (or liquid) at a room temperature (e.g. Al.sup.+, Ga.sup.+, P.sup.+, As.sup.+, Sb.sup.+) is often used in dependence on the types of ions.
In the case of the hitherto known ion source apparatus shown in FIG. 1, the solid or liquid specimen 3 to be ionized is placed in a vaporizing furnace 1 and vaporized under heating by a heater 4, wherein the gas resulted from vaporization is introduced into the ionization chamber 2 through a second gas feeding pipe 11 for ionization of the gas.
FIG. 2 is a sectional view showing a main portion of a hitherto known filament heating type ion source. In the figure, equivalent or same parts as those shown in FIG. 1 are denoted by like reference numerals.
Referring to FIG. 2, a vaporizing furnace 1 having an inner space to be charged with a solid or liquid specimen 3 is heated by means of a heater coil 4 disposed around the outer periphery of the furnace 1. The heating temperature is monitored by means of a thermometer which may be constituted by a thermocouple or the like, to be controlled and maintained at a predetermined value.
The gas resulted from vaporization of the solid or liquid specimen 3 is introduced into an ionization chamber 2 through a gas feeding pipe 11. A filament 19 is disposed under tension within the ionization chamber 2. Upon heating of the filament 19 through electrical energization, thermal electrons 20 are emitted into the ionization chamber 2 and collide with the specimen vapor or gas particles, which are thus ionized.
Although not shown in FIG. 2 for simplification of the illustration, the ionization chamber is externally applied with a magnetic field to impart the thermal electrons 20 with rotational movement for increasing the probability of collision.
The ions thus produced are drawn out under the action of the extracting electrode (not shown) in the form of an ion beam.
As will now be understood, the hitherto known ion sources including the microwave discharge type ion source are of the structure in which only one vaporizing furnace is provided. Such structure involves various problems mentioned below.
Since raw materials or specimens for different types of ions (e.g. As.sup.+ and P.sup.+) can not be charged simultaneously, it is impossible to produce the ions of different types in a continuous manner.
As the consequence, when different types of ions are required to be produced, the vaporizing furnace and the ion source must be cooled down after production of ions of a certain type. Subsequently, another type of raw material has to be placed in the vaporizing furnace which is of course accompanied with destruction of vacuum. After the vacuum in the vaporizing chamber has been restored, the vaporizing furnace is again heated up to produce another type of ion beam. During this process, the apparatus has to be shut down for a long period which usually amounts to two hours or more, which in turn means degradation in the operation efficiency as well as working ratio of the ion source.
Further, when the solid or liquid raw material within the vaporizing furnace has been exhausted before implantation of ions in a desired amount is attained, the procedure mentioned above has to be taken to again charge the vaporizing furnace with the raw material, involving the problem of degraded operation efficiency and working ratio.